Does anyone have some books they could refer me to that would sum up the history/efficacy of torture when being used to gather intel? Or links that are good sources will work as well.
These may be of some use to you, although some go beyond what you're asking for. Due to my area of research, most of these are concerning the recent American experience with torture:
Alfred J. McCoy, Torture and Impunity: The U.S. Doctrine of Coercive Interrogation (University of Wisconsin Press, 2012).
Alfred J. McCoy, A Question of Torture: CIA Interrogation, from the Cold War to the War on Terror (Holt, 2006).
Laurel E. Fletcher and Eric Stover, The Guantanamo Effect: Exposing the Consequences of U.S. Detention and Interrogation (University of California Press, 2009).
Marjorie Cohn, The United States and Torture: Interrogation, Incarceration, and Abuse (NYU Press, 2012).
Jess Bravin, The Terror Courts: Rough Justice at Guantanamo Bay (Yale University Press, 2013).
Marnia Lazreg, Torture and the Twilight of Empire: From Algiers to Baghdad (Princeton University Press, 2007).